This invention relates to a distance measuring system using an electric distance meter, and particularly relates to a reflector used therein.
An electric distance meter is broadly used to measure a distance between two distal positions. FIG. 1 is a schematic view showing a principle of a distance measuring system using the electric distance meter. In order to detect the distance of two positions P1 and P2, the electric distance meter 110 is located at one position P1. Further, a reflector unit 120, which has a reflector 121 (such as a prism), is located at the other position P2. The electric distance meter 110 emits modulated light (such as infrared rays) to the reflector unit 120. The emitted light is reflected by the reflector 121 and returns to the electric distance meter 110. The electric distance meter 110 detects a phase difference of the emitted light and returned light, to calculate the distance between the positions P1 and P2. Such an electric distance meter can be coupled with an goniometer (such as an electronic theodolite), to constitute a measuring system known as a `total station`.
FIG. 2A and 2B are a front view and a sectional view of the conventional reflector unit 120. The reflector unit 120 includes the reflector 121, and a U-shaped support 134 which supports the reflector 121. The reflector 121 is a so-called corner cube which reflects the incident light in parallel to the direction of the incidence. The reflector unit 120 further includes e leveling board 140 mounted on a tripod and a stage 137 adjustably provided on the leveling board 140. The U-shaped support 134 is mounted to the adjustable stage 137 via a shaft 135.
As shown in FIG. 1, the direction of the light emitted from the electric distance meter 110 must be directed to the reflector unit 120. For this purpose, the electric distance meter 110 has a sighting telescope 111 through which an operator can observe the reflector unit 120. With this, the operator is able to correctly direct the electric distance meter 110 toward the reflector unit 120, while observing the reflector unit 120 through the sighting telescope 111.
In order to focus the sighting telescope 111 on the reflector unit 120, the sighting telescope 111 generally includes an autofocus unit of a phase detection focusing type (which is generally used in a single lens reflex camera).
However, when the autofocus unit operates, the sighting telescope 111 may be focused on a virtual image of the electric distance meter 110 reflected by the reflector 121. In such case, the reflector unit 120 is out of focus and can not be seen from the operator. Thus, the operator can not correctly direct the electric distance meter 110 toward the reflector unit 120.